Everything compiles with no errors. Basically I copied the Histogram example from the Qwt site and slimmed it down for speed. a few other tweaks were made as well, nothing major though. I read in values from a .txt file where every line is the y value. the x value is the line number. For small text files with small numbers, I am able to clearly see the graph (ie x.interval = 1-10, y.interval = 1-10 respectively, so the text file would just have the 10 numbers in it, separated by a carriage return) but when i try to expand to larger files with 10,000 entries, i get a flat bar across the bottom and the last few entries have infinite y values. when i tried a file with 1 million entries, it crashed (probably having to do with using int). below is the code. any help or suggestions will be happily welcomed!

histogram.cpp
Qt Code: 
  1. #include "histogram_item.h"
  2.  
  3. class HistogramItem::PrivateData
  4. {
  5. public:
  6.     int attributes;
  8.     QColor color;
  9.     double reference;
  10. };
  11.  
  12. HistogramItem::HistogramItem(const QwtText &title):
  13.     QwtPlotItem(title)
  14. {
  15.     init();
  16. }
  17.  
  18. HistogramItem::HistogramItem(const QString &title):
  20. {
  21.     init();
  22. }
  23.  
  24. HistogramItem::~HistogramItem()
  25. {
  26.     delete d_data;
  27. }
  28.  
  29. void HistogramItem::init()
  30. {
  31.     d_data = new PrivateData();
  32.  
  33.     setZ(20);
  34. }
  35.  
  36. double HistogramItem::baseline() const
  37. {
  38.     return d_data->reference;
  39. }
  40.  
  41. void HistogramItem::setData(const QwtIntervalData &data)
  42. {
  43.     d_data->data = data;
  44.     itemChanged();
  45. }
  46.  
  47. void HistogramItem::setColor(const QColor &color)
  48. {
  49.     if ( d_data->color != color )
  50.     {
  51.         d_data->color = color;
  52.         itemChanged();
  53.     }
  54. }
  55.  
  56. QwtDoubleRect HistogramItem::boundingRect() const
  57. {
  58.     QwtDoubleRect rect = d_data->data.boundingRect();
  59.     return rect;
  60. }
  61.  
  62.  
  63. int HistogramItem::rtti() const
  64. {
  65.     return QwtPlotItem::Rtti_PlotHistogram;
  66. }
  67.  
  68. void HistogramItem::draw(QPainter *painter, const QwtScaleMap &xMap, 
  69.     const QwtScaleMap &yMap, const QRect &) const
  70. {
  71.     const QwtIntervalData &iData = d_data->data;
  72.  
  73.     painter->setPen(QPen(d_data->color));   
  74. //    const int x0 = xMap.transform(baseline());
  75.     const int y0 = yMap.transform(baseline());
  76.  
  77.     for ( int i = 0; i < (int)iData.size(); i++ )
  78.     {
  79.             const int y2 = yMap.transform(iData.value(i));
  80.               int x1 = xMap.transform(iData.interval(i).minValue());
  81.               int x2 = xMap.transform(iData.interval(i).maxValue());
  82.             drawBar(painter, Qt::Vertical,
  83.                 QRect(x1, y0, x2 - x1, y2 - y0) );
  84. }
  85. }
  86.  
  87. void HistogramItem::drawBar(QPainter *painter,
  88.    Qt::Orientation, const QRect& rect) const
  89. {
  90.    painter->save();
  91.  
  92.    const QColor color(painter->pen().color());
  93. #if QT_VERSION >= 0x040000
  94.    const QRect r = rect.normalized();
  95. #else
  96.    const QRect r = rect.normalize();
  97. #endif
  98.  
  99.    const int factor = 125;
  100.    const QColor light(color.light(factor));
  101.    const QColor dark(color.dark(factor));
  102.  
  103.    painter->setBrush(color);
  104.    QwtPainter::drawRect(painter, r.x()+1 , r.y()+1,
  105.      r.width()-3, r.height()-3);
  106.  
  107.    painter->restore();
  108. }
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histogram.h
Qt Code: 
  1. #ifndef HISTOGRAM_ITEM_H
  2. #define HISTOGRAM_ITEM_H
  3. #include <qglobal.h>
  4. #include <qcolor.h>
  5. #include <qapplication.h>
  6. #include <qmainwindow.h>
  7. #include <qwt_counter.h>
  8. #include <qtoolbar.h>
  9. #include <qlabel.h>
  10. #include <qlayout.h>
  11. #include <qfile.h>
  12. #include <stdio.h>
  13. #include <iostream>
  14. #include <fstream>
  15. #include <QMessageBox>
  16. #include <QMainWindow>
  17. #include <QTextStream>
  18. #include <QTextOStream>
  19. #include <stdlib.h>
  20. #include <qpen.h>
  21. #include <qwt_plot.h>
  22. #include <qwt_plot_grid.h>
  23. #include <qwt_plot_marker.h>
  24. #include <qwt_interval_data.h>
  25. #include <qstring.h>
  26. #include <qpainter.h>
  27. #include <qwt_painter.h>
  28. #include <qwt_scale_map.h>
  29. #include "qwt_plot_item.h"
  30.  
  32. class QString;
  33.  
  34. class HistogramItem: public QwtPlotItem
  35. {
  36. public:
  37.     explicit HistogramItem(const QString &title = QString::null);
  38.     explicit HistogramItem(const QwtText &title);
  39.     virtual ~HistogramItem();
  40.  
  41.     void setData(const QwtIntervalData &data);
  42.     const QwtIntervalData &data() const;
  43.  
  44.     void setColor(const QColor &);
  45.     QColor color() const;
  46.  
  47.     virtual QwtDoubleRect boundingRect() const;
  48.  
  49.     virtual int rtti() const;
  50.  
  51.     virtual void draw(QPainter *, const QwtScaleMap &xMap, 
  52.         const QwtScaleMap &yMap, const QRect &) const;
  53.  
  54.     void setBaseline(double reference);
  55.     double baseline() const;
  56.  
  57.     enum HistogramAttribute
  58.     {
  59.         Auto = 0,
  60.         Xfy = 1
  61.     };
  62.  
  63.     void setHistogramAttribute(HistogramAttribute, bool on = true);
  64.     bool testHistogramAttribute(HistogramAttribute) const;
  65.  
  66. protected:
  67.     virtual void drawBar(QPainter *,
  68.         Qt::Orientation o, const QRect &) const;
  69.  
  70. private:
  71.     void init();
  72.  
  73.     class PrivateData;
  74.     PrivateData *d_data;
  75. };
  76.  
  77. #endif
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main.cpp
Qt Code: 
  1. #include "histogram_item.h"
  2.  
  3.  
  4. int main(int argc, char **argv)
  5. {
  6.     QApplication a(argc, argv);
  7.  
  8.     QwtPlot plot;
  9.     plot.setCanvasBackground(QColor(Qt::white));
  10.     plot.setTitle("Histogram");
  11.  
  12.     QwtPlotGrid *grid = new QwtPlotGrid;
  13.     grid->enableXMin(true);
  14.     grid->enableYMin(true);
  15.     grid->setMajPen(QPen(Qt::black, 0, Qt::DotLine));
  16.     grid->setMinPen(QPen(Qt::gray, 0 , Qt::DotLine));
  17.     grid->attach(&plot);
  18.  
  19.     HistogramItem *histogram = new HistogramItem();
  20.     histogram->setColor(Qt::darkCyan);
  21.  
  22.     const int numValues = 10000;
  23.  
  24.     QwtArray<QwtDoubleInterval> intervals(numValues);
  25.     QwtArray<double> values(numValues);
  26.  
  27.     QStringList lines;
  28.     QFile file( "C:\\path\\to\\file\\file.txt" );
  29.     int count[10000];
  30.     if(file.open(QFile::ReadOnly))
  31.     {
  32.        QTextStream stream(&file);
  33.        QString line;
  34.        int i = 1;
  35.        line = stream.readLine();
  36.  
  37.        while(!line.isNull())
  38.        {
  39.          line = stream.readLine();
  40.          if(line.toInt() != 0)
  41.          {
  42.          lines += line;
  43.          count[i-1] = line.toInt();
  44.          i++;
  45.         }
  46.          else{
  47.              lines+=line;
  48.              i++;
  49.              }
  50.  
  51.        }
  52.     }
  53.     file.close();
  54.  
  55.     double pos = 0.0;
  56.     for ( int i = 0; i < (int)intervals.size(); i++ )
  57.     {
  58.         int width = 1;
  59.         int value = count[i];
  60.         intervals[i] = QwtDoubleInterval(pos-.5*double(width), pos + .5*double(width));
  61.         values[i] = value; 
  62.  
  63.         pos += width;
  64.     }
  65.  
  66.     histogram->setData(QwtIntervalData(intervals, values));
  67.     histogram->attach(&plot);
  68.  
  69.     plot.setAxisScale(QwtPlot::yLeft, 0.0, 10000.0);
  70.     plot.setAxisScale(QwtPlot::xBottom, -1, pos);
  71.     plot.replot();
  72.  
  73. #if QT_VERSION < 0x040000
  74.     a.setMainWidget(&plot);
  75. #endif
  76.  
  77.     plot.resize(800,600);
  78.     plot.show();
  79.  
  80.     return a.exec(); 
  81. }
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